1. Field of the Invention
The present invention relates to a fluorescent material for use in, for example, a field emission display (FED) and a manufacturing method therefor, and more particularly to an improvement in a yttrium oxide fluorescent material activated with europium.
2. Description of the Related Art
Particles of a yttrium oxide fluorescent material activated with europium (Eu)(Y.sub.2 O.sub.3 :Eu) are mainly prepared by two methods. That is, a method is employed with which powder of a yttrium oxide and powder of an europium oxide are mixed with each other by a ball mill. Then, powder of the mixed materials is baked. Another method has the step of dissolving powder of the yttrium oxide and powder of the europium oxide by using nitric acid. Then, the materials are co-precipitated as an oxalate which is then baked.
The latter method is an advantageous method which permits a fluorescent material excellent in light emitting efficiency to be obtained.
A variety of fluorescent materials are employed in displays of television receivers. Usually, the particle of the fluorescent material is several .mu.m (3 .mu.m to 10 .mu.m).
In recent years, a variety of display units have been developed. From a viewpoint of reducing the thickness of the display, a plasma display (PDP), a field emission display (FED) and an electro-luminescence display (ELD) have attracted attention.
The structure of the FED cannot raise the accelerated voltage. Thus, there arises a problem in that the fluorescent material having a particle size of about several .mu.m for use in the conventional CRT cannot sufficiently perform excitation. The reason for this lies in that the low acceleration voltage cannot enable electron beams to reach portions at which particles emit light.
Therefore, a nano-crystal fluorescent material having a nano particle size has been studied.
The nano-crystal fluorescent material is able to raise the probability of existence of light emitting portions adjacent to the surfaces of the particles. Thus, excitation is permitted even at the low acceleration voltage. That is, the nano-crystal fluorescent material is suitable for a fluorescent material for the FED unit.
For example, the FED unit can be excited at the low acceleration voltage as described above. Therefore, the distance for which electron beams are introduced is too short to sufficiently excite the FED by the fluorescent material particles each having the particle size of several .mu.m.
Therefore, a fine nano-crystal fluorescent material suitable for the FED and precise CRT has been required.
Under the foregoing circumstances, the applicant of the present invention has disclosed a method of preparing nano-crystal fluorescent material of the yttrium oxide having europium by using liquid-phase reactions.
The yttrium oxide fluorescent material activated with europium (Y.sub.2 O.sub.3 :Eu) has a problem of unsatisfactorily low intensity of color (chromaticity). To raise the intensity of color (that is, to raise the chromaticity), the concentration of Eu must be raised. When the concentration of Eu is raised, the transaction of Eu results in the brightness being lowered (the foregoing phenomenon is called "concentration extinction").
Therefore, the yttrium oxide fluorescent material activated with europium (Y.sub.2 O.sub.3 :Eu) is not permitted to raise the concentration of Eu to improve the intensity of color. Thus, improvement of the foregoing problem is an important factor. Also raising of the brightness is required.